why are they?

[honda400ex2003]

So i was sitting in a heat transfer class today taking notes when my teacher draws 2 diagrams on the board. as shown below. One with fins inside a tube and one with fins outside the tube. He then asks which one is better at cooling with ALL things being the same. the things that are the same are liquid going through the pipe inside, air outside, same materials for fins, no friction from fins inside causing any flow issues, nothing. Which is better- fins inside or out? and why is one more efficient than the other? I am interested to see if anyone knows the exact answer to the question at hand and chooses the correct one. Have fun guys, lets hear em. :D steve

inside the tube

[jcs003]

outside the tube. the air flow across the fins dissapate the heat. its a simple heat exchanger. the mass flow is greater with outer oriented fins. i did problems like this in thermodynamics and heat transfer when i was in college.

[honda400ex2003]

nice, any other explanations for which one would be better? i figured we could have a decent discussion about the exact reason why it is. Anyone know about the heat transfer coefficients of the air and the water and why it makes a difference there? JCS pretty much got most of it covered. lol congratulations, steve

[ZSK]

AS JCS said, cooling with the fins outside is increased. The cooling can be increased or decreased depending on the surface area exposed by the fins. Simply put; more surface area, more cooling capacity. Take it a step further and look at the surface of the fins, compare a polished fin to one that is cast in place or a fin that has been media blasted.

[flyboy1294]

Originally posted by ZSK
AS JCS said, cooling with the fins outside is increased. The cooling can be increased or decreased depending on the surface area exposed by the fins. Simply put; more surface area, more cooling capacity. Take it a step further and look at the surface of the fins, compare a polished fin to one that is cast in place or a fin that has been media blasted.

Cast in place would be the most efficient at heat displacement.

The reason for this is that there are almost microscopic craters that give the cast fin more surface area, thus allowing for better heat displacement.

When you polish the fin, you smooth out these craters.

Surface area is the name of the game essentially.

Am I right? :D

[honda400ex2003]

air has a lower heat transfer coefficient that makes the efficiency of the fins go up when on the outside.

the efficiency is proportional to 1/h
where h is the heat transfer coefficient of the air. the water has a higher coefficient thus when the fins are inside the water doesn't have the efficiency that the air does on the outside. that is the answer we got in heat transfer today lol steve

[honda400ex2003]

Originally posted by flyboy1294
Cast in place would be the most efficient at heat displacement.

The reason for this is that there are almost microscopic craters that give the cast fin more surface area, thus allowing for better heat displacement.

When you polish the fin, you smooth out these craters.

Surface area is the name of the game essentially.

Am I right? :D

i believe you are :D that would be my guess at least lol steve

[Ralph]

This went a lot smoother then the last HT thread.

That being said, the fins on the inside still increase the rate of the heat transfer to the tube. But the air is generally less conductive so that is going to be your bottleneck, thus you want this area to be the largest.

[honda400ex2003]

lol i thought it would be an interesting topic. it seemed that some of the people in class saw it the other way, that the fins inside would be better :eek2: thats why i thought maybe it would be a decent discussion. too easy for us atvers though lol. gotta love radiators and oil coolers. :D steve